Actually according to WM's site it is a 6oz differential in fill. Also I would think the fill used in the full down collar in Ultralite is included in the fill weight. And that collar would affect the overall rating of bag in that specific placement of that amount of down.
We are talking about too many variables. Humidity, wind, shelter, metabolism, weight, body mass, gender, height, hat, socks, genetics, hygiene, dinner, timing, length of sleep, mood, health....comfort is always subjective. I think a reasonable range is the best anyone should offer.